1. Field of the Invention
The present invention relates to medical instruments and methods for performing eye surgery to correct irregularities of the cornea. More particularly, the present invention relates to mechanical instruments known as microkeratomes, and related surgical methods for performing lamellar keratotomies.
2. The Related Art
The first microkeratome for performing corneal resections was developed in 1962 by the Doctor Jose I. Barraquer, and is shown generally in FIG. P1. This microkeratome includes a guide ring which is fixed to an ocular globe, or eyeball, with the aid of a partial vacuum applied through the guide ring. The guide ring immobilizes the ocular globe, maintains the tension of the globe, and aids in regulating the diameter of the corneal resection. A portion of the microkeratome called a cutting head is supported within a channel in the guide ring for guided linear movement of the microkeratome across the ring by the surgeon. The cutting head carries a cutting blade that is oscillated by a motor-driven eccentric transverse the channel as the instrument is moved through the cutting path defined by the channel. The cutting head carries a removable, lower planar member that compresses the ocular globe ahead of the oscillating blade, to permit the blade to cut a lamella having a lower surface that is parallel to the surface of the cornea that is compressed by the planar member. The planar member is interchangeable with similar planar members of differing thicknesses, so as to vary the thickness of the resectioned corneal "disc."
Numerous variations on the Barraquer microkeratome have been made since 1962, including the apparatus that is the subject of U.S. Pat. No. 4,662,370 assigned to Carl-Zeiss-Stiftung of Germany. The '370 patent describes a microkeratome having interchangeable inserts with convex, concave, and planar surfaces that engage and compress the cornea for producing a corneal resection of predetermined form and curvature. The inserts are set within a stationary planar member that is fixed to the guide ring. The cutting blade is moved through a cutting path parallel to the planar member defined by a gap between the planar member and the guide ring, and oscillates transverse the path.
While apparently effective to permit resections of corneal lenticula, the apparatus of the '370 patent lacks means for controlling, or automating the rate of movement by the cutting head across the guide ring, and is therefore prone to binding up in the corneal tissue, or otherwise producing imprecise resections under unsteady progress by the surgeon's hand. Furthermore, there is no apparent means for changing the depth or thickness of the corneal resection. Also, this apparatus is limited to use in lamellar keratectomies (excision of a corneal section), as opposed to lamellar keratotomies (incision through the cornea).
The problem of controlled movement across the guide ring has been addressed by the instrument described in U.S. Pat. No. 5,133,726, which has been reissued as U.S. Pat. No. Re 35,421, to Luis A. Ruiz and Sergio Lenchig G. The '726 and '421 patents disclose a microkeratome, shown in FIG. P2, having a gear transmission assembly for moving the instrument through the cutting path at a controlled rate of speed. The gears are driven by the same motor that drives the cutting blade and engage a track atop the guide ring. Thus, the automated transmission system is an improvement over the instrument of the '370 patent, but in practice it has been found that the weight of the motor in the instrument produces a large moment through the handle of the device. This moment, coupled with the forward positioning of the gear that engages the guide ring track, causes the rear surface of the cutting head to bind in its engagement with the guide ring. At best, this results in uneven travel by the instrument during the surgery and unnecessary pressure fluctuations within the eye. At worst, such binding can cause irregular cutting of the cornea that produces leucoma, or the induction of an astigmatism.
The relatively recent technological development of intrastromal refractive surgery led to the creation of instruments and methods for performing incomplete lamellar temporo-nasal keratotomies, which leave a peripheral residue of corneal tissue uncut to act as a "nasal hinge." The nasal hinge permits the corneal disc to be lifted for exposure and carving of the stromal layer, such as by a laser. The use of a laser to perform stromal carving in association with an incomplete lamellar keratotomy is referred to as "Laser Intrastromal Keratomileusis" ("LASIK").
In similar fashion to the original Barraquer device, the microkeratome of the '726 and '421 patents include a forward planar member in the lower portion of the cutting head that is interchangeable with similar planar members of varying thicknesses. For the planar member to be interchangeable, however, a slotted portion of the cutting head extends substantially forward of the cutting blade to receive the planar member. This, and the fact that the transmission gears are positioned outside the cutting head, result in a fairly large surface area, or "footprint" for the instrument. The large footprint restricts the manner in which the microkeratome can be used, and generally requires that it be moved across the cornea from the temporal region adjacent the eye, producing the vertical nasal hinge when performing incomplete lamellar keratotomies. The vertical nasal hinge has at least two deficiencies. First, the corneal disc resulting from the LASIK, or other procedure, will be vertically displaced after surgery, and/or pleated to some extent by the opening and closing of the upper eyelid. Second, the formation of a vertical nasal hinge on the corneal disc increases the likelihood of accidental ablation of the hinge during the correction of an astigmatism, which is typically performed with horizontal cutting motions across a major diameter of the cornea.
The large surface area of the planar member, or plaque, described in the '726 patent is designed to substantially compress the entire cornea at any one time. Such action produces unnecessarily high intraocular pressure, which unduly stresses the eye and could result in complications during surgery.
Further problems with known microkeratomes have been observed in performing resections on patient's having small eyes. The smaller ocular structure, particularly the peripheral structure, of such patients presents great difficulty during a lamellar keratotomy, since a portion of the surgical instrument may collide with the ocular structure and cause surgical accidents. This problem persists because, in spite of all efforts to perform lamellar keratotomy with more reliable instruments, the physical size of the instruments and the required surface area that the instrument must occupy during a lemellar keratotomy increase the likelihood that some portion of the microkeratome structure will encounter the patient's ocular structure.
Another problem with known systems, such as the microkeratome described in U.S. Pat. No. 5,624,456, relates to the manner in which the cutting head is brought into contact with the corneal surface. More specifically, the microkeratome of the '456 patent induces movement of a cutting blade through a flat plane which is defmed, by necessity, to clear the patient's ocular structure. For purposes of discussion, this plane may be considered to be a horizontal plane since the patient's head will be more or less horizontal during the procedure. In order for the cutting blade to intersect the cornea, the eye must be pulled outwardly over the ocular structures so as to place a portion of the cornea above the horizontal plane. This creates a risk of suction loss between the surgical guide ring and the eye during the operation, with potentially severe consequences.
Other problems in the related microkeratome art include the requirement of interacting drive gears which must be constantly maintained for smooth operation, and the limited options for placement of the suction orifice on the surgical guide ring. The latter problem is a result of the need to leave open a clear path or guideway in the guide ring for passage of the cutting head and cutting blade, since the cutting blade is carried in a flat, horizontal plane as described above.
It is an object of the present invention to address one or more of the shortcomings described herein, as well as others.